The present invention relates to an apparatus for molding a recorded disc and particularly to such an apparatus which minimizes the extent of the flash formed adjacent the center hole of the disc.
A high packing density recorded disc, such as a video disc, in which the packing density is in the order of 10,000 grooves per inch (3,937 per cm), has grooves which are much finer in dimension than an audio recorded disc and rotates at much higher speed. The grooves in a high packing density disc are about 2.7 microns in width, as compared to 50-60 microns for an audio disc, and are in the order of 4,000 Angstroms in depth. Also, the high packing density disc rotates at about 450 rpm as compared to 33 rpm for an audio disc. Because of the fine dimensions of the groove and the high speed of rotation of the disc, the disc must be supported in a stable position to insure proper tracking of the stylus in the groove. To help achieve the required stability, the disc is provided with a relatively large center hole, about 1.3 inches (3.3 cm) in diameter and is seated in the player on a truncated conical spindle.
A method generally used to make the disc is compression molding. It has been found desirable to form the center hole in the disc as it is being molded. An apparatus suitable for forming the center hole in the disc as it is being molded is shown and described in copending application for U.S. Letters Patent of Michael Lee McNeely, Ser. No. 093,012, filed Nov. 9, 1979, "METHOD AND APPARATUS FOR PRODUCING DISC RECORDS HAVING MOLDED-IN CENTER HOLES", and assigned to the assignee of the present application which is incorporated herein by reference. A problem with this apparatus is that it leaves the record with flash about the center hole of the record at both of the surfaces of the record. At one surface the flash projects radially inwardly from the edge of the center hole and at the other surface the flash projects normal to the other surface from along the edge of the center hole. Thus, no matter which way the disc is mounted on the spindle, the surface of the spindle will contact the flash so as to interfere with the proper seating of the disc on the spindle. Thus, the flash can result in unstable support of the disc during the playing of the disc.
Copending application for U.S. Letters Patent of R. W. Chambers et al, Ser. No. 187,161, filed Sept. 15, 1980, entitled "APPARATUS FOR MOLDING A RECORDED DISC", and assigned to the assignee of the present application which is incorporated herein by reference, describes a molding apparatus in which the center hole forming pin and its mating center hole plate are designed to move the flash away from the edge of the center hole of the disc. This alleviates the problem of improper seating of the disc on the spindle. However, it is still desirable to maintain the extent of such flash at a minimum, if not completely remove it, particularly the flash on the surface of the disc.
The flash at the surface of the disc is formed at the interface between the center hole forming pin and the center plate in which the pin is mounted. The extent of the flash is determined by the fit of the pin with the center plate. When the pin has a good fit in the center plate, the extent of the flash is at a minimum. As the surface of the pin or the passage in the center plate become worn because of the movement of the pin in the center plate, the clearance between the pin and the center plate become larger and the extent of the flash increases. Thus, in order to maintain the extent of the flash below a desired maximum, the center hole pin and/or the center plate must be replaced periodically as they become worn, which is an added expense in making the disc. It has been found that this occurs after making about 5000 discs. Therefore, it would be desirable to have a molding apparatus wherein the extent of the flash is maintained at a minimum without the need of replacing the center hole forming pin or the center plate.